This invention relates to the production of polyurethane and/or polyurea elastomers, more particularly to the production of such elastomers in a reaction injection molding process.
Polyurethane elastomers are generally prepared by reacting a polyisocyanate with a relatively high equivalent weight material having active hydrogen atoms, or mixture of such materials having active hydrogen atoms and a relatively low equivalent weight compound or mixture thereof which also contains a plurality of active hydrogens (chain extender). In the commercial production of many such elastomers, the foregoing components are mixed and reacted in a reaction injection molding (RIM) process. In the RIM process, a stream containing the active hydrogen-containing materials and a second stream containing the polyisocyanate are injected under high pressure through a mixing zone and into a mold where curing takes place.
Typically, the complete mixing and filling operation takes place in about 0.5 to about 10 seconds, depending on the particular materials used and the size and configuration of the mold. The reactivity of the materials used must therefore be such that the mold can be filled before the reactants gel. In addition, it is desirable to demold the elastomer as rapidly as possible in order to produce the maximum number of molded parts per mold per period of time. Thus, it is desired that the materials used cure rapidly so the elastomer can be removed from the mold in a short period of time.
In order to remove the elastomer from the mold, it must have enough physical integrity to withstand the stresses encountered during the actual removal process. For example, in demolding an elastomer, it is often necessary to pull, bend, twist or otherwise distort it in order to effect release from the mold. In many cases, the complex geometry of the mold requires that the elastomer be stretched in the demolding process. Accordingly, the elastomer must be able to withstand these actions without tearing. This ability is usually referred to in the art as the "green strength" (or hot tear strength) of the elastomer.
For the forgoing reasons, the materials used in preparing RIM elastomers and the method of preparing the elastomer, must be chosen for their processing characteristics as well as for the properties they impart to the product elastomer.
In preparing elastomers via a RIM process, it is now standard practice to use a "one-shot" process. In the one-step process, the polyisocyanate is reacted simultaneously with the high equivalent weight active hydrogen-containing material and the chain extender. The polyisocyanate is either not prereacted with any of the active hydrogen-containing materials, or is reacted with only a minor portion thereof to form a so-called "quasi-prepolymer". This process is used because it enables the practitioner to balance the weights of the streams which are injected into the mixing head. Because of mechanical limitations in some RIM equipment, it has not been possible to mix streams of significantly different weights. The one-step process also has the advantage of requiring one less process step, since the prepolymer need not be formed.
The major drawback of the one-step process is that it is difficult to prepare high modulus elastomers in this manner. For example, to prepare high modulus polymers for applications such as automobile body panels, it has been found necessary to employ a filler such as milled or flaked glass. As a result, the impact strength and elongation of the elastomer suffer greatly.
In the two-step process, the polyisocyanate is reacted in a first step with all or a major portion of the high equivalent active hydrogen-containing material to form a prepolymer. This prepolymer is then reacted in RIM equipment with the chain extender and any remaining high equivalent weight material to form the elastomer. Unfortunately, it has heretofore been difficult to process these two-step systems. Due to the fast reactivity of the chain extenders, particularly amine-terminated chain extenders such as diethyltoluenediamine (DETDA) it has been particularly difficult to completely mix the components and fill the mold before gelling occurs. In addition, elastomers prepared according to a two-step process have often been extremely brittle at demold.
Due to the disadvantages encountered with the one-step process and the previously known two-step processes, it would be desirable to provide a process for preparing a polyurethane and/or polyurea polymer in which an elastomer having excellent physical properties can be obtained, and in which the difficulties of two-step processes are minimized or overcome.